1. Field of the Invention
The present invention relates to a novel force transducer, particularly for ballistic pressure measuring, which includes a force transmission element subjected to the pressure to be measured at one of its frontal areas, a sensor element held under pretension between the force transmission element and a basic receiver component, and an outside sleeve surrounding the outside of the force transmission element and subjected to the pressure to be measured at its frontal area. Additionally, a method for the production of the novel force transducer is disclosed.
2. Description of the Prior Art
Different methods and measuring arrangements are known for determining the level and the development in time of the pressures occurring in a cartridge case. A known method provides for the use of "copper crushers" or copper crusher gauges. For this purpose, a hole is drilled into the cartridge case and the combustion gases are led into a cylinder surrounding a piston. The piston presses on a cylindrical copper piece which, depending on the level of the pressures of the combustion gas, is more or less compressed. The expansion of the diameter or the reduction in the length of the copper piece gives an indication of the level of the pressure in the cartridge case. This method serves the purpose of comparison testing; it gives not an indication of the respective prevailing peak pressures since the compression not only depends on the peak pressure but also on the induction period of the pressure.
A better indication of the development of the pressure is obtained when the pressure in the cartridge case into which a hole has been drilled is transmitted to a piezoelectric high pressure transducer. The entire development in time of the pressure can then be recorded by an oscillograph so that the peak pressure as well as the pressure rise and pressure drop can be precisely quantitatively determined.
Both of the above methods have a systematic deficiency in that a hole must be drilled into the cartridge case for this purpose. Apart from the gas losses in the cartridge case or cartridge chamber caused by the hole drilled into it, the necessary connecting duct to the pressure transducer involves additional volume and the pressure build-up is lessened because of the larger volume behind the cartridge case. Thus, the measured peak pressures do not correspond to the actual pressures in the cartridge case.
Another known method provides for the measuring of the pressure directly in the drilled hole of the cartridge chamber into which the cartridge case is introduced. Drilling a hole into the cartridge case can thus be avoided. Apart from the fact that a measuring error also occurs with this method because of the additional volume, this method has the disadvantage that no measuring signal is received prior to the time when the projectile leaves the drilled hole of the cartridge chamber. Therefore, the pressure build-up in the interior of the cartridge case is not determinable during the first phase of the combustion and the sudden release of the measuring duct to the pressure transducer by the projectile indicates too steep a pressure rise.
Another piezoelectric pressure transducer for ballistic pressure measuring is shown in U.S. Pat. Nos. 3,886,792 and 3,960,018. This device rests on the shaft of the cartridge case by means of a diaphragm so that the forces resulting from the elastic and plastic expansion of the cartridge case are transmitted to the transducer. The transducer is installed in a hole drilled in the cartridge chamber for this purpose and the diaphragm in contact with the outer surface of the cartridge case is designed in such a manner that it forms a continuation of the part of the hole drilled in the cartridge chamber which is cut away by it. The pressure in the cartridge case expands the case somewhat so that the force is transmitted to the piezoelectric measuring quartz elements arranged in the pressure transducer through the cartridge case and the diaphragm on the pressure transducer. The electrical charge of these quartz elements is led to an electrometer or charge amplifier. This known method has the advantage that the measuring system does not interfere practically with the combustion process and the flow conditions and, additionally, it is possible to also provide for serial measurements since no special measuring preparations are necessary for the individual shots. However, this known arrangement has the disadvantage that it is very difficult to exactly adapt the pressure recording front portion of the pressure transducer to the hole drilled in the cartridge chamber or at the outside circumference of the cartridge case.
Considered from the point of view of measuring technique, the connection between the gas pressure in the cartridge case and the released piezoelectric charge should be as linear as possible. The piezoelectric effect is actually very linear per se but the pressure is not effected in a proportional force in the cartridge case due to the complicated transmission to the measuring quartz elements. Since a cartridge case must always have a certain play in the cartridge chamber so that it can be inserted or thrown out without difficulty, there exists already a certain discontinuity from this point of view when the cartridge case is expanded by the combustion pressure and rests closely against the inside of the borehole in the cartridge chamber. Up to that point in time, the gas pressure has an effect only in the inside of the cartridge case but not on the pressure transducer. Therefore, the first phase is not measured prior to the point when the cartridge case rests against the front portion of the pressure transducer. The pressure indication prior to this point in time is zero. In order to obtain the effective pressure, the initial cartridge expansion pressure must be added to the measured pressure which, however, does not present any problem from the point of view of the measuring technique.
As has been shown by experience, the cartridge case is always plastically deformed when the slot is fired and remains expanded after the shot. Therefore, when the front portion of the pressure transducer which rests on the cartridge case is part of the borehole in the cartridge chamber and projects into it, the curve Q=f(P) recorded by the pressure transducer initially rises steeply because, initially, the cartridge case presses only on the pressure transducer and drops thereafter as soon as the cartridge case starts resting against the borehole in the cartridge chamber with higher pressures. However, when the front portion of the pressure transducer is somewhat recessed in comparison with the cartridge chamber borehole, a gap remains between the cartridge case and the front portion of the pressure transducer. The cartridge case bulges only under higher pressures in the area of the pressure transducer to such an extent that it contacts it. In this case, there exists a more or less large range in which no measuring signal is transmitted by the transducer depending on the size of the recessing of the front portion of the pressure transducer. The exact adaptation of the shape of the front portion of the pressure transducer to the respective configuration of the cartridge case is not only of importance for the size of the range over which the pressure transducer responds but also for the reproducibility of the measurement since even slight inhomogenities can lead to discontinuities in the force distribution on the measuring quartz elements.
Due to these circumstances, it is proposed in the aforementioned U.S. patents that the front portion of the pressure transducer formed by the diaphragm be adapted to the configuration of the cartridge chamber borehole by means of washers. Apart from the fact that such adjustment measures are bothersome and undersirable for the customer, the required contact exactness of a few micrometers is not given so that certain edge pressures and such still remain. This has the result that no pre-established calibration curve can be made available to the user of the pressure transducer which can be used at the point of application for the evaluation of the measurements without having to take any additional measures. On the contrary, the known piezoelectric pressure transducers for ballistic measurements always require that the user carry out a static calibration with a gas or a liquid for the respective firearm, i.e. to establish an individual calibration curve. Besides the mentioned circumstances, this is also necessary because the rigidity of a pressure transducer is always lower than that of the cartridge chamber so that the pressure transducer and the cartridge chamber suffer different deformations with the expansion of the cartridge case, i.e. the diaphragm of the pressure transducer is recessed vis-a-vis the outer circumference of the cartridge chamber borehole. The developed depression has the result that the cartridge case no longer rests completely against the front face of the diaphragm as assumed in the calibration but the contact surface is more or less reduced depending on the size of the depression. The reduced force on the measuring quartz elements resulting from the change in the contact conditions is expressed as a deviation from the linear path of the calibration curve which differs from case to case. As a result, a calibration curve adapted to a particular application must always be established in the case of the known pressure transducers which, however, can sometimes involve considerable errors due to the kind of static application of the calibration pressure.
Furthermore, the measuring quartz elements are arranged very closely to the pressure-subjected diaphragm in the case of the known piezoelectric pressure transducers which, besides supplying a poor force distribution on the measuring quartz elements, has the disadvantage that they are exposed to a high thermal load.